In South Carolina. Had cover on for 4 months. Air temp all this week in the high 70's. Took cover off two days ago on Monday. Opened to a crystal clear pool and a water temp of 55F. FC was at 1.5 still so I brought up to 4FC, 0CC, brushed and had robot cleaner going. Noticed yesterday about 5 - 6 dusty brown spots about a quarter in size. Did a OCLT last night and went from 3 to 2.5FC still 0CC and brushed them away this morning. Just went back at lunch and they are back. Water too cold for salt generator so adding chlorine manually. Do I need a SLAM or just hit it hard with chlorine for a day or two?